Adjustable port arrangement for the high-pressure ends of fluid pumps and motors of the rotary screw type



Oct. 27, 1953 RATHMAN 2,656,972

ADJUSTABLE PORT ARRANGEMENT FOR THE HIGH-PRESSURE ENDS OF FLUID PUMPS AND MOTORS OF THE; ROTARY SCREW TYPE Filed Jan. 31. 1949 4 Sheets-Sheet l I N VEN TOR.

()ctu27, 1953 e. RATHMAN. 2,656,972

ADJUSTABLE FORT ARRANGEMENT FOR THE HIGH-PRESSURE ENDS I OF FLUID PUMPS AND MOTORS OF THE ROTARY SCREW TYPE Filed Jan. 51, 1949 4 Sheets-Sheet 2 I N VENTOR.

Oct. 27, 1953 G. RATHMAN 2,656,972

ADJUSTABLE PORT ARRANGEMENT FOR THE HIGH-PRESSURE ENDS 0F FLUID PUMPS AND MOTORS OF THE ROTARY SCREW TYPE Filed Jan. 31-. 1949 4 Sheets-Sheet 3 29.65. -zo/jge 7 I N VEN TOR.

Oct. 27, 1953 RATHMAN 2,656,972

ADJUSTABLE PORT ARR EMENT FOR THE -HPRESSURE ENDS OF FLUID PUMPS AND MOTORS OF THE R RY SCREW TY Filed Jan. 51 1949 4 She Sheet 4 Patented Oct. 27, 1953 ADJUSTABLE PORT ARRANGEMENT FOR THE HIGH-PRESSURE ENDS OF FLUID PUMPS AND MOTORS OF THE ROTARY SCREW TYPE Gilbert Rathman, Union, N. J., assignor, by mesne assignments, to Dresser Industries, Inc., a corporation of Pennsylvania Application January 31, 1949, Serial No. 73,784

1'7 Claims.

This invention relates to rotary screw pumps and motors of the type comprising intermeshing, helically ribbed rotors which are adapted for transporting or compressing fluids, especially air and other gases, or for operation as expansible fluid engines, and is more particularly directed to the improved forms of such apparatus disclosed in my copending applications, Serial No. 511,083, filed November 20, 1943, and Serial No. 707,384, filed November 2, 1946, now Patents Nos. 2,450,310 and 2,511,878, dated February 1, 1949, and June 20, 1950, respectively.

Although the devices of these prior applications and those of the present invention are reversible in operation so as to function as either pumps or motors, the following description will, in the interest of simplicity, designate the devices as pumps and use phraseology generally applicable to air compressors. By so doing, however, it is not intended to limit the scope of the invention to pump or compressors, although it is in this field that it would appear to find its greatest utility at the present time.

The pumps disclosed in the above identified applications, upon which the present invention is an improvement, are characterized by the combination of a pair of coacting rotors, each having a single helical rib or thread, which rotate in interineshed relation in a casing continuously open at the inlet end to a source of low pressure fluid and having intermittent connection at the other end with high pressure Or discharge conduit, and means for controlling communication between the discharge conduit and the fluid transporting and compressing space defined by the rotor ribs in such a manner that the fluid is confined in said space until the desired pressure is built up, whereupon, once in every revolution of the rotors, the space is opened to the discharge conduit through a passageway or port which faces, and lies within the axially projected cross section of, the threaded portion of one of the rotors. The latter rotor may conveniently be called the discharge or driving rotor, while the other is known as the mating or driven rotor.

In the first or the earlier filed applications, the discharge of the fluid from the compression space is controlled by a valvular device comprising two apertured coacting valve elements, one being fixed to the casing and the other being connected to the discharge rotor and turning therewith to open the high pressure end of said rotor to the discharge conduit once in each rotation. In the second application, the escape of the fluid into the discharge conduit is controlled by a fixed discharge port of predetermined configuration and volumetric capacity which is opened and closed periodically by the rotation of the discharge rotor itself, the end of the rotor rib or thread servin to cover and uncover the port as the rotor turns.

Both of these prior pumps are capable of maintaining a constant flow of gas or other fluid into the discharge line at a predetermined pressure without objectionable surging or other variable pressure conditions in the fluid at the discharge end of the pump. While it is possible to vary the compression ratio of these prior devices by changing the size and configuration of either the passageway in the rotating valve element of the first application or the discharge port in the second application, it is necessary to stop the pump and take it out of service for this: purpose, since the change involves replacement of either the rotating valve element or that in which the discharge port is formed.

It is therefore the principal object of the present invention to provide, in a device of the class described, novel means for varying the compression ratio which canbe actuated, either manually or automatically, while the device is in operation.

Another object is to provide a pump of the rotary screw type with improved means for controlling the point in the cycle of the device at which the high pressure end of the compression space defined by the rotor is opened to communication with the discharge line, which means permit variation of the time of fluid discharge without the necessity for an interchange of parts or removal of the device from service.

A further object is to provide a screw type gas pump or compressor having a built-in compression ratio which may be varied over a substantial range without varying the efficiency of the device, and wherein the output pressure is determined by the circumferential or angular position of a discharge port of fixed size and shape which is rotatably adjustable about the axis of the discharge rotor of the pump.

Still another object is to provide a compressor of the character described with means for preventing a further build-up of pressure therewithin during each cycle after the discharge port has once been opened and closed, particularly when said port is so positioned as to provide a relatively low compression ratio.

These and other objects will appear more fully upon consideration of the detailed description of certain embodiments of the invention which follows. Although only two specific forms of apparatus are described and illustrated in the accompanying drawings, it is to be expressly understood that these drawings are for the purpose of illustration only and are not to be construed as defining the scope of the invention, for which latter purpose reference should be had to the appended claims.

Referring now to the drawings, wherein. like reference characters indicatetlike parts through-- out the several views:

Fig. 1 is a plan view of one form of screw pump or compressor embodying the invention with certain parts shown in section, the section. being taken substantially on the line l-l in Fig. 2;

Fig. 2 is a side elevation of the device shown in Fig. 1 with certain parts shown in section, the

section being taken substantially on the line 2-2 in Fig. 1;

Fig. 3 is a cross section of the same construction taken substantially on the line 3-4;: in Fig. 2, looking inwardly toward the high pressure or discharge ends of the rotors and showing'the ad'- justable discharge port in the position which produces the maximum discharge pressure and compression ratio;

Fig. 4 is a cross sectional view similarto Fig. 3, but wherein the discharge port is'so positioned as to produce a lower discharge pressure than in the caseof Fig. 3;

Fig. 5 is a cross sectional view of the device of Figs. 1 and 2 taken substantially on the line 5 in Fig. 1' but turned'through $0" for easier com: parison. with Figs. 3 and thisview looking in the opposite direction to Figs. 3 and 4 showing the position of the relief. passageway which prevents an excessive build-up of pressure. when the discharge port is so located as to provide a rela tively low compression ratio, as in the case of Fig. 4;

Figs. 6, 'l, 8, 9 and 10 are views similar to Figs. 1, 2, 3, at and 5, respectivelyof a second embodiment of the invention.

Referring now to Figs. 1-5, the pump structure shown therein comprises a horizontally extending casing 2i open at both ends and having a laterally projecting fluid intake conduit 22 adjacent one end and, adjacent the other end, a similar fluid discharge conduit 23. As shown: best in Fig. 5',

the main body of the casing 2| is a-shaped in cross section, consisting of a pair of hollow cylinders 24 and 2?; in partially overlapped relation with their axes parallel and offset from one another, the discharge end of the casing being enlarged as indicated in Figs. 3 and: 4' to form a substantially elliptical fluid discharge chamber 25. The open ends of the casing'Ei are normally closed by a pair of hollow heads 21' and 28 which are removably secured to the flanged ends of the casing in any suitable manner, as by bolts (not shown). Housed in each of the casing heads 2T and 28 are a pair of combination radial and end thrust bearings 29 which" rotatablysupport the shafts it and 3| of a pair of intermeshed, helically threaded rotors 32' and 33 adapted, in known manner, to transport air, gas or other fluid from the intake conduit 22, through the casing 2! and out through the discharge conduit 23, and'at the same time to compress the fiuid prior to discharge.

The shaft 30 of the rotor 32, hereinafter re ferred to as the driving or discharge rotor, projects outwardly of the casing head 2'! at the intake end of the casing, and is adapted to be connected to a suitable source of power for turning the driving rotor in the direction indicated by the arrows appearing at the right of Figs. 1 and 2. The shaft 3% of the second rotor 33, hereinafter referred to as the driven or mating rotor, is drivably connected to-the shaft 3'0 of'tlie driving rotor by'meansof gears 36 housed in the casing head 2? which are adapted to rotate both screws at the same speed.

The rotors 32 andv 33 are keyed to their respectiveshafts iii-and 31 for rotation therewith, and are held against axial movement along said shafts, to the'right. as viewed in Figs. 1 and 2, by stop. shoulders. and 38 formed on the shafts. The driving rotor is provided with a single helical. thread or. rib 3? having at least one full turn or convolu... .n, the peripheral surface of which has running fit the upper portion of the casing. 2! formed by the cylinder 2 The driven rotor 33 is similarly provided with a thread or rib 38 of the same length as the thread 37 andhaving a running fit in the lower cylinder of the casing.

As shownbest in Fig. 2, each of the threadsor ribs 3? and 3;; is concave in longitudinal cross section at one side; while the opposite side is con vex. For example, the left side of the driving thread 3"? is concave, as viewed in Fig. 2, while the right side flit is convex, or substantially so. The driven thread 38 is oppositely formed'in that its left side is convex, or practically convex, while the right side 52 is concave. The result is that the two rotors are so formed that the similarly shaped sides of the threads face one another, and that the-concave side 1553 of the driving thread 310i the discharge rotor faces the fluid discharge chamber 255. It will al'sobe noted that, in this preferred form of rotorconstruction, which is the subject matter of the previously mentioned Patent No. 2,511,873,the-thread 3? of the discharge rotor is relatively thin in an axial directionin comparison withthe thread aaof the driven-rotor, and that the groove between the turns of the thread 3? is correspondingly wider than'the groove M of the driven rotor.

Ey'reference toFig. 5, it Will'beseenthat' each rotor exhibits in any section perpendicular to its axis a compound curved.- outline consisting of' an outerperipheral surface of cylindrical curvature: conforming to the curvature of the inside wall of the casing in which it rotates, an inner rootor dedendum circle t5 concentric with the associated rotor. shaft, a concave side 5?. connecting one end of the outer peripheral surface and one end of the exposed portion of the root circle 45, and a convex. side 48 connecting the other end. of the peripheral surface with the root circle. The concave side of the thread is of epicycloidal shape, while the convex side ii-l has a curvature substantially that of an Archimedes-n spiral;

The stop shoulders 35 andfib on the rotor shafts are located, as shown in Figs. 1 and 2, a substan tial distance. inwardly of the. casing from. the casing head 2! so as to provide a relatively large fluid. intake chamber Gtwhich is in permanently open',.direct communication with the fluid intake conduit22 andalso communicates with the intake end of the fluid transporting and compressing space defined by the rotor threads 37 and 38 and the surrounding wall of the casingZ I. In order to control the discharge of the fluid which ispicked up by the screws from the intake chamber 49 and transported axially through the casing 2|, the discharge end of" the latter is provided with a novel-arrangement of elements which-permits the fluid to be discharged from the space 5!] into the discharge chamber 26 and conduit 23 once in every revolution of the rotors, and also permits the time of said discharge to be varied with respect to the angular positions of the rotors without dismantling the pump or even stopping its operation.

In the embodiment illustrated, there are removably fitted into the discharge end of the cas ing 2! two hollow castings 5i and 52 having outer walls 53 and respectively, which abut casing head 2% and inwardly extending sleeves or bosses 55 and it through which pass the rotor shafts 32 and 33, respectively. The inner end of the sleeve 55 of the upper casting 5! is reduced in diameter at 51 to provide a bearing for a rotatably adjustable valve member 55 in which is formed a discharge or cut-ofi port 5!! adapted to register once in each revolution of the rotors with another port til formed in a disc Bl which is fixed to and rotates with the driving rotor shaft 38. When the ports 5!; and (it overlap, the fluid in the space 5% can escape into the channel a: formed by the hollow interiors of castings 5i and 52 and thence into the discharge chamber 26 and outlet When the ports 55 and Bil are not in registry, the fluid is retained in the space 51 and is compressed in a manner hereinafter described.

The valve member 53 has a central hub 63 which is journaled on the reduced diameter por tion 5? of sleeve an annular web or body por tion 64 in which the cut-01f port 59 is located, and outer rim portion which is provided with gear teeth lid on its outer periphery the purpose of rotatably adjusting the valve member about the axis of the driving rotor shaft 3! so as to vary the circumferential position of the port 59. In the embodiment illustrated, the gear teeth Sill extend around the entire rim 65 in the form of a worm wheel and are in engagement with a worm M which is fixed to a shaft 58 journaled in and extending laterally outwardly of the enlarged elliptical portion of the casing 2i surrounding the discharge chamber 25. The outer end of theworm shaft E8 is provided with a suitable handwheel 6! or other means for rotating the worm and thereby adjusting the valve member. The inner vertical surface of the web or body portion of the cut-on valve member EB abuts the outer or left-hand surface of the disc valve 6! as viewed in Figs. 1 and 2, and is held thereagainst both by the shoulder it on the sleeve 55 of casting 5! which abuts the outer end of the hub $3 and by the pressure of the fluid in the discharge chamber 25 and the communicating channel iii The rim 65 of the valve member is recessed as indicated at H to receive a ring '52, li-shaped in cross section, which engages the left-hand end of the portion of the casing 2! formed by the upper cylinder 24 and the upper arcuate edge It of the inner vertical wall Hi of casting 52, and which assists in properly positioning and centering the valve member.

The inner vertical wall is of the lower casting 52 abuts the discharge end of the thread 38 of the driven rotor 33 and forms an imperforate barrier to the escape of fluid into the channel E2 and discharge chamber 26 from that portion of the fluid transporting and compressing space 5B which is defined by the unrneshed portion of the driven rotor thread 38 and the lower portion of the casing 2|, i. e., that portion of the driven rotor groove 44 which is unoccupied by the driving rotor thread 31. The cut-off valve member 58, which is substantially coextensive in cross-sectional area with the axially projected area of the driving rotor thread 31, similarly blocks the escape of fluid from the upper portion of the space 59 at all times other than when the port 60 of the disc valve 6! registers with the cut-off port 59 in the valve member 58. The length of time during which the fluid may be discharged from the space 50 depends upon the size and configuration of the coacting ports 59 and 68, while the point in the cycle of operation of the pump at which this discharge occurs is determined by the angular position of the cut-off port 59 about the axis of the driving rotor shaft 30.

Referring now to Figs. 3-5, the cut-off port 59 therein shown extends over approximately onequarter of the area of the web or body portion 64 of valve member 58, and is bounded by two concentric arcs a-b and c-d of different radii having the axis of the driving rotor shaft as their center, and by two other arcs u.-c and b-cl having the same radius as the rotors. The circumferential or angular length of the port 58, which is approximately in. the construction illustrated, determined the limits between which the compression ratio of the pump may be varied by adjustment of the port about the axis of the driving rotor. The position of the cut-off port 59 shown in Fig. 3 represents one extreme of its adjustability, the position in which the compression ratio is a maximum (approximately 4 to l in the present instance). In this maximum pressure position, the left-hand end of the cut-off port defined by the arc a-c is substantially parallel to the upper left-hand portion of the periphery of the driven rotor. By rotating the valve member 53 in a clockwise direction as viewed in Fig. 3, by means of the handwheel worm shaft 68 and worm El, the position of the cut-off port 55 may be varied so as to permit discharge of the compressed fluid from the rotors at an earlier point in the cycle of operation, and thereby produce a lower final pressure and compression ratio. For example, when the cut-oil port 59 occupies the position shown in Fig. 4-, representing the opposite limit of its adiustability in the structure illustrated, the compression ratio is reduced to about 2 to 1. if desired, suitable stops may be provided to prevent the valve mem ber 55 from being rotated beyond the intended limits of its movement represented by the positions of the port 53 shown in Figs. 3 and 4.

As will be apparent from Figs. 3 and 4 which show the two extreme positions to which port 59 may be adjusted, said port always lies wholly within the axially projected outline or area of that portion of the driving rotor thread 3'. which is out of mesh with the driven rotor thread 33; i. e., it never lies within the axially projected outline or area of the intermeshing portions of the rotor threads represented by the lens shaped area below the shaft to enclosed within the lines 45 and 73 in Fig. 5.

In the embodiment of the invention illustrated, the port 6G formed in valve disc ti is lens shaped and is of the maximum permissible area, this area being equal to the axially projected area of the intermeshing portions of the rotor threads. Although the disc port so can be reduced below this maximum size, in which event it is possible to lengthen the cut-oil? port 59 in a clockwise direction as viewed in Figs. 3 and 4 and thereby I attain a lower; minimum pressure, it cannot be madelarger without resulting in a. leak. through from'the high. pressure side to the low pressure side of the pump.

Because of the fixed circumferential, or arouate length of the cut-off port 59, whenever the latter is in any position other than that illustrated in Fig. 3 (wherein the maximum compression ratio is provided) the discharge of fluid from the space 56 is cut ofi; due to rotation of the disc port 6% out of registry with the cuton port, prior to completion of the rotational cycle of the pump. For example, when the cutoff port 53 is in the minimum compression position shown in Fig. 4, wedge-shaped pocket having a projected area a--c-ef is formed be-' tween the concave leading side of the driving thread 3i and the barrier formed by the imperforate portion of the web E8 of the valve member 53, which pocket has a zero thickness along the line a-c and a maximum thickness along. the line e f, the latter thickness depend ing on the helix angle of the thread. As the screws continue to rotate, the volun e of this pocket approaches zero with a consequent increase in pressure of the fluid trapped therein. In order to save power and prevent possible damage to the apparatus, means are provided for relieving the fluid pressure which would thus be built up in this pocket by venting the fluid directly into the discharge chamber 26 or the discharge conduit 23. In the embodiment of the invention illustrated, this is accomplished by providing a suitably dimensioned by-pass opening through the wall of the casing it closely adjacent the discharge or high pressure end thereof and leading into the discharge conduit 23, said opening being located in the portion of the casing formed by the upper cylinder just above the intersection of the latter with the lower cylinder 25, as shown in Fig. 5. With this construction, the fluid remaining in the pocket a-ce-f after the discharge path through the, disc port so and cut-oii port 59 has been closed is lay-passed into the discharge conduit without affecting the efficiency of the pump in any manner.

The method of operation of the device disclosed in Figs. 1-5 is the same in principle as that of the apparatus of the above identified Patent No. 2,460,319, the principal difference being that, in the pump of the present invention, the compression ratio may be varied, within the limits permitted by the size and shape of the cut-off port 5Q, by simply rotating the valve member 58 and thereby adjusting the position of the cut-off port about the axis of the driving rotor. Assuming that air is supplied to the intake conduit 22 and that the rotors 32 and 33 are being driven from a suitable source of power connected to the driving rotor shaft 38, the air entering the intake chamber 49 will flow into. the low pressure or right-hand ends of the rotor grooves 43 and 44 and will be advanced by the rotor threads 3'! and 38 toward the left as viewed in Figs. 1 and 2. As the rotors turn, the transporting andv compressing space deformed by the grooves 43 and. 44 gradually decreases in volume so that the air trapped therein between the advancing rotor threads and the barrier formed by the valve member 58 and the inner vertical wall M of the casting 52 is placed under pressure. Since the discharge end of the driven rotor groove 44 is always out off from direct communication with the discharge chamber '26 by the wall 14, the air compressed in this groove is transferred, to. the portion of compressionspace formed by the driving" rotor.

groove 43 for final, compression and discharge. The discharge orcut-oif port 59 is so positioned that the disc portBll comes into registry therewith just as the rotors have advanced to the point where the air is compressed to the desired degree, whereupon continued rotation of the rotors and disc valve 6|, withthe ports 59 and in direct communication with one anothen'causes the air to flow out of the compression space 50, through the channel Gland discharge chamber 26 into the discharge conduit or outlet 23. If it is desired to.

vary the discharge pressure produced by the pump, it is only necessary to adjust the position of the cut-off port 59 by means of the handwheel 69. When the cut-off port 59 is in any position other than that which provides the maximum discharge pressure obtainable (Fig. 3), the by-pass opening 15 serves as a relief port for preventing the production of an undesired pressure in the terminal'or discharge end of the driving rotor groove l3v after communication between the cutoif port 59 and disc port 50 has been shut off.

Referring now to Figs. 6-10, the pump structure illustrated therein is of substantially the same construction as that of Figs. l-5 except for omission of the disc valve 6| and its port 60 and a change in the configuration of the discharge or cut-off port which is designated by the-reference numeral. 59 in Figs. 6-10. Omission of the disc valve also results in a reduction in the thickness of the inner vertical wall 14 of the lower casting 52, since said wall no longer has to provide a bearing surface for the disc valve as in the previously described embodiment. Inasmuch as the common features of, the two structures. will be apparent from the drawings, it is unnecessary to describe in detail those portions of Figs. 6-10 which find their counterparts in Figs. 1-5.

The discharge or cut-off port 59 of Figs. 6-10 differs from the corresponding. element of the first embodiment only in the shape of the curve which defines the right-hand end of the port as viewed in Figs. 8 and 9., Instead of being a circular arc of the same radius as the rotors, this curve 12-12 is of the same shape as the convex edges of the rotor threads when viewed in axial cross section as shown in Fig. 10, i. e., a close approximation of an Archimedean spiral. With a cut-off port of this configuration and of the angular length shown in the drawing, amaximum compression ratio of approximately 2 to 1 is obtained with the port in the position shown in Fig. 8. In Fig. 9, the port is so positioned as to provide a lto 1 compression ratio, in which event the device functions merely as a fluid transport apparatus,- rather than a compressor. It will be understood. particularly by reference to the disclosure of the above mentioned Patent No. 2,511,878, which is directed to the same basic structure as that embodied in Figs. 6-10 hereof, that higher compres sion ratios may be obtained by shortening the angular length of the port 59' and thereby reducing both the area of the port and the length of time during which the compressed fluid may be discharged from the compression space into the discharge chamber and conduit constituting the compressor outlet.

The operation of the embodiment of Figs. 6-10 is substantially the same as that of the previously described device'of Figs; 1-6, except that opening and closing ofthe cut-off port 59' is controlled directly by the end 01 the driving rotor thread 31, instead of by the disc valvefi'l and port 60.

There are thus provided by the present invention improved forms of screw type pumps or compressors wherein the compression ratio or discharge pressure produced by the device may be readily controlled and varied during operation by a simple adjustment of the position of a discharge or cut-oil? port of predetermined size and shape, and wherein provision has also been made for relieving any undesired pressure that may be built within the pump following delivery of a charge of the desired pressure when the cut-off port is so positioned as to provide a compression ratio less than the maximum attainable.

While two specifically difierent devices embodying the invention have been described and illustrated in the accompanying drawings, it will be obvious that the invention is not limited to the particular structures shown, but is capable of a variety of mechanical embodiments. For example, although the means for rotatably adjusting the valve member carrying the discharge or cut-01f port have been shown as a manually operated worm and worm wheel arrangement, it is obvious that other forms of actuating mechanism, such as a rack and pinion, bevel gears or the like, may be substituted for the specific arrangement illustrated, and that the adjustment may be made automatically as well as manually, as by the use of any suitable pressure responsive actuator to which the output pressure of the pump, or any other desired pressure, might be applied as the controlling medium. It should also be apparent that, since the cut-01f port valve member need have only a limited rotational movement, it is unnecessary that the gear teeth extend completely around the periphery thereof, although such an arrangement is advantageous in that it permits location of the worm or equivalent driving element at a number of different positions around the casing. The intake and discharge conduits likewise may be located at various points about the casing dilferent from the laterally projecting positions illustrated in the drawings. It is a1so to be understood, as previously pointed out, that while the above description has been directed primarily to pumps and compressors of the positive displacement type, the same structures may be used as rotary engines of the expansible fluid type by simply reversing the direction of flow of the fluid therethrough.

Various other changes, which will now become apparent to those skilled in the art, may be made in the form, details of construction and arrange ment of parts of the herein disclosed devices without departing from the inventive concept. Reference is therefore to be had to the appended claims for a definition of the limits of the invention.

What is claimed is:

1. In a rotary screw type pump or motor embodying a pair of helically ribbed rotors adapted to rotate in intermeshed relation in a casing continuously open at one end to a conduit in which a relatively low fluid pressure prevails and having restricted communciation at the other end with a conduit wherein a higher fluid pressure exists, means for controlling communication between the high pressure conduit and the high pressure end of the fluid compression or expansion space defined by the rotor ribs and the easing comprising a member normally fixed with respect to said casing forming an imperforate Wall perpendicular to the axis of said rotors which prevents direct flow of fluid between the high pressure end of one of said rotors and the high wall-forming portion complementary to the wall formed by said first member and facing the high pressure end of the second rotor, said second member being rotatably adjustable relatively to said casing and to said second rotor about the axis of the later, a single port in the wall-forming portion of said second member having continuous communication with the high pressure conduit and communicating with the high pressure end of said second rotor during at least a portion of each revolution of the latter, and means for rotatably adjusting said second member about the axis of said second rotor to vary the circumferential position of said port.

2. In a rotary screw type pump or motor embodying a pair of helically ribbed rotors adapted to rotate in interineshed relation in a casing continuously open at one end to a conduit in which a relatively low fluid pressure prevails and having restricted communication at the other end with a conduit wherein a higher fluid pressure exists, means for controlling communication between the high pressure conduit and the high pressure end of the fluid compression or expansion space defined by the rotor ribs and the casing comprising a member normally fixed with respect to said casing forming an imperforate wall perpendicular to the axis of said rotors which prevents direct flow of fluid between the high pressure end of one of said rotors and the high pressure conduit, a second member having an annular wall-forming portion complementary to the wall formed by said first member and substantially coextensive in area with the axially projected area of the ribbed portion of the second rotor, said second member being rotatably adjustable relatively to said casing and to said second rotor about the axis of the latter, a single port in the wall-forming portion of said second member having continuous communication with the high pressure conduit and communicating with the high pressure end of said second rotor during at least a portion of each revolution of the latter, and means operable from outside said casing for rotatably adjusting said second member about the axis of said second rotor to vary the circumferential position of said port.

3. In a rotary screw type pump or motor embodying a pair of helically ribbed and grooved rotors adapted to rotate in intermeshed relation in a casing continuously open at one end to a conduit in which a relatively low fluid pressure prevails and having restricted communication at the other end with a conduit wherein a higher fluid pressure exists, means, for controlling communication between the high pressure conduit and the high pressure end of the fluid compression or expansion space formed by the rotor grooves comprising a member normally fixed with respect to said casing forming an imperforate wall perpendicular to the axis of said rotors which prevents direct flow of fluid between the high pressure end of the groove of one of said rotors and the high pressure conduit, a second member having a wall-forming portion complementary to the wall formed by said first member and facing the high pressure end of the second rotor, said member being rotatably adjustable relative to said casing and to said second rotor about the axis of the latter, a single port in the wall-forming portion of said second member opposite the unmeshed ribbed portion of said second rotor, means providing direct communication at all times between said port and the high pressure conduit, means for establishing communication aeeaeva between said-port and .the high pressure end of the grooverof said seoondrotorfor alimited time during each revolution of the latter, and means operable during rotation of said rotors for rotatably adjusting said second member about the axis'of said second rotor to vary the circumferential position of said port.

4. A screw .type fluid pumpcomprising a pair of intermeshing, helically threaded rotors, a a casingfor said rotors having a fluid'inlet adjacent one end and a fluid outlet adjacent the other end,said .fiuid inlet being in direct communication with the intake end of the fluid pumping space definedby the rotor threads and the casing,and means forproviding restricted communication between the discharge end of said fluid pumping space and the fluid outlet from :said casing including a partition perpendicular to the axes of said rotors at the discharge end of saidfiuidpumping space, one portion of said partition being normally fixed relative to the casing and forming an imperforate barrier to the direct discharge of fluid to the fluid outlet from the portion of said fluid pumping space defined by the unmeshedportion of the thread or" one of said-rotors, another portion of said partition being movable about the axis of said second rotor, a single port in-the movable portion of said partition lying wholly within the axially projected outline of the unmeshed threaded portion of said second rotor, said port havingdirect communication with the-fluid outlet and communicating with the discharge end of the fluid pumping space during at least a portion of each rotation of said rotors, and means for moving said movableportion of the partition about the axis of said second rotor to vary the circumferential position of said port.

5. A screw type fluid .pump comprising a pair of intermeshing, helically threaded and grooved rotors, a casing for said rotors having .a fluid inlet adjacent one 'end and a fluid discharge chamber at theother end, said fluid-inlet being in direct communication with the intake end of the fluid pumping space formed by the rotor grooves, and means for providing restricted communication between the discharge end of said fluid pumping space and the fluid discharge chamber'including a partition in said discharge chamber perpendicular to the axes of' said rotors at the discharge end of said fluid pumping space, one portion of said partition being normally fixed to the casing and another portion being movable about the axis of one of said rotors, the fixed portion of said partition having an imperiorate wall abutting the discharge end of the second rotor and having an area substantially equal to the axially projected area of the unmeshed threaded portion of said second rotor, whereby direct discharge of fluid from the groove of said rotor into the discharge chamber is prevented, the movable portion of said partition facing the discharge end orthe first-rotor and having an area substantially equal to the axially projected area of the entire threaded portion thereof, a single port in the movable portion of said partition lying wholly within the axially projected outline of the unmeshed threaded portion of said first rotor, said port having direct communication with the fluid discharge chamber and communicating with the discharge end of the groove in said first rotor during at least a portion .of each rotation of said rotors, and :means for moving said movable portion of the partition 12 about-the axis of said first rotor to vary the circumferential position of said port.

6. A screw type'fluid pump comprising aapair oi intermeshing, helically threaded rotors, a casing for said rotors having a fluid'inlet adjacent one end and a fiuidoutlet adjacent the other end, said fluid inlet being in direct-communication with the intake end of the fluid pumping space defined by therotor-threads and the casing, and means for'jproviding restricted communication between the discharge end of said fluid pumping space and the fluid outlet from said casing including-apartition-perpendicular to the axes of said rotors at the discharge end of said fluid pumping space, .one portion of said partition being normally fixed to the casing and another portion being .movable about the axis of one of said rotors, -a single port in the movable portion of said partition lying wholly within the axially projected outline of the unmeshed threaded portion of said last named rotor and having direct communication with thefluid outlet, the remainder of said partition being imperforate, means for establishing communication between the discharge end of the fluid pumping space and said port once during each rotation of said rotors comprising an apertured disc fixed to said last named rotor at the discharge end thereof and shutting the movableportion of said partition, and means forrnoving said movable portion of the partition aboutthe axis of said last named rotor during rotation of the rotors to vary the circumferential position of said port.

7. A screw type fluid ump comprising a pair of intermeshing, helically threaded and grooved rotors, a casing for said rotors having a fluid inlet adjacent one end and a fluid outlet adjacent the other end, said fluid inlet being in direct communication with the intake en'dof the fluid pumping space formed by the rotor grooves, and means for providing restricted communication between the discharge end of said fluid pumping space and the'fluid outlet -.from said casing including a partition perpendicular to the axes of said rotors andin abuttinglengagement with the discharge ends thereof, oneportion of said partition being normally fixedto the casing and another portion being. movableabout the axis of one of said rotors, a singleport in the movable portion of said partition lying wholly within the axially projected outline .of the unmeshed threaded portion of said llast named rotor and having direct communication at all-times with the fluid outlet, the remainder of said partition being imperiorate, said port having communication with the discharge .end of the groove of said last named rotor whenever said port is uncovered by the end oithe thread of said last named rotor, and means for moving said movable portion ofthe partition about the axis of said last named rotor during rotation of the rotors to vary the circumferential position of said port.

8. A rotary screw type pump according to claim 4 wherein the means for moving the movable portion of the partition to vary the position of the port includes an actuating member operable from outside the casing of the pump.

9. A rotary screw type pump according to claim 4 wherein variation in the, position of the port in the movable portion of the partition varies the compression ratio of the pump, and which includes a relief passageway leadingfrom apo'int in the fluid pumping space closelyedja- 13 cent the partition to the fluid outlet for venting fluid from said space to the outlet when said port is in any position other than that wherein the maximum compression ratio is obtained.

10. A rotary screw type pump according to claim 4 wherein the pump casing is formed by a pair of overlapping cylinders producing a casing substantially B-shaped in cross section, and which includes a relief passageway through the casing wall at a point closely adjacent the partition and the plane of intersection of said overlapping cylinders, said passageway connecting the fluid pumping space in the casing with the fluid outlet so as to by-pass fluid from said space to the outlet.

11. A rotary screw type pump according to claim 6 wherein the sides of the port in the movable portion of said partition are defined by a pair of circular arcs of diiferent radii concentric with the axis of the rotor about which the port is movable, while the ends of the port are concave and are defined by a second pair of circular arcs of the same radius as said rotor.

12. A rotary screw type pump according to claim 7 wherein the sides of the port in the movable portion of said partition are defined by a pair of circular arcs of different radii concentric with the axis of the rotor about which the port is movable, one end of the port is concave and is defined by a circular arc of the same radius as said rotor, while the other end is convex and is approximately an Archimedean spiral in shape.

13. In a screw compressor having a casing and a pair of helically threaded screws adapted to rotate in intermeshed relation with a running fit in said casing and to compress a fluid during its passage through the casing prior to discharge therefrom, means for controlling the discharge of the compressed fluid from the casing comprising a wall across the discharge end of the casing including two coacting elements having a combined area perpendicular to the axes of the screws equal to the cross-sectional area of the threaded portions of said screws, one of said elements being annular in form and substantially coextensive in area with the axially projected outline of the threaded portion of one of said screws, means for supporting said element for rotational movement about the axis of said screw and in close proximity to the discharge end thereof, the second element being normally fixed relative to the casing and forming an imperforate barrier abutting the discharge end of the second screw except for the area thereof which meshes with the first named screw, a single discharge port formed in said movable element through which the compressed fluid is discharged from the casing during at least a portion of each revolution of the screws, and means operable from outside the compressor casing for rotatably adjusting said movable element about the axis of said first named screw to vary the circumferential position of said port and thereby vary the pressure of the fluid discharged therethrough.

14. A screw compressor comprising a casing,

a pair of helically threaded rotors adapted to rotate in intermeshed relation with a running fit in said casing and to compress a fluid during its passage through the casing prior to discharge therefrom, each of said rotors having a thread which is concave on one side and convex on the other side, said rotors being so intermeshed that the concave side of the thread of the first rotor faces the concave side of the thread of the second rotor and also faces the discharge end of the casing, and means for controlling the discharge of the compressed fluid from the casing including a wall across the discharge end of the casing perpendicular to the axes of said rotors, the portion of said wall facing the discharge end of said first rotor being movable about the axis of said rotor, the remainder of said wall being fixed relative to the casing and forming an imperforate barrier to the discharge of fluid from the casing,

a single discharge port formed in the movable portion of said partition and lying wholly within the axially projected outline of the unmeshed portion of the thread of said first rotor, and means for rotatably adjusting said movable portion of the partition about the axis of said first rotor to vary the circumferential position of said port and thereby vary the pressure of the fluid discharged therethrough.

15. A screw compressor according to claim 14 wherein the movable portion of the partition is annular in form and substantially coextensive in area with the axially projected outline of the threaded portion of said first rotor.

16. A screw compressor according to claim 14 wherein communication between the fluid compression space within the casing and said discharge port is controlled by an apertured disc to said first rotor at the discharge end thereof and abutting the movable portion of said partition.

17. A screw compressor according to claim 14 wherein the discharge end of the thread of said first rotor abuts the movable portion of said partition and controls communication between the fluid compression space within the casing and said discharge port.

GILBERT RATHMAN.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 165,805 Disston July 20, 1875 960,992 Motsinger June 7, 1910 960,994 Motsinger June 7, 1910 2,058,817 Northey Oct. 27, 1936 2,266,820 Smith Dec. 23, 1941 2,287,716 Whitfield June 23, 1942 2,460,310 Rathman Feb. 1, 1949 2,504,230 Smith Apr. 18, 1950 2,511,878 Rathman June 20, 1950 2,578,196 Montelius Dec. 11, 1951 FOREIGN PATENTS Number Country Date 316,670 Great Britain 1931 

